Patent GB-1529314-A (GB)
Title: Communication apparatus
IPC: G06Q40/04
Filed: 1975-10-31
Abstract: 1529314 Selective signalling GENERAL SIGNAL CORP 31 Oct 1975 [18 Nov 1974] 45542/75 Heading G4H A communication apparatus for communicating between a central station and a plurality of separately addressable remote stations over a duplex communication link includes a transmitter and receiver in each station, and digital computer means for controlling the central station transmitter and responsive to the central station receiver, the computer means including a first ("poll") timer for each remote station with means to set each timer to a value related to the maximum desired delay between successive communications with the associated remote station and monitor the timers to detect expiry, there being means to cause transmission to a remote station whose timer has expired. As disclosed, the computer (CPU) communicates in both directions over the duplex link via a modem controller and modem, a message to a remote station consisting of remote station address, control word(s) and check word(s), the latter added to the message by the modem controller, and a message from (overlapping that to in time) a remote station consisting of the remote station's address (used for address check comparison, no-compare preventing transmission of the check word(s) of the message to the remote station, thus aborting this message), indication word(s) and check word(s), the check word(s) being used in error checking at the modem controller. The modem controller interrupts the CPU to obtain successive words during transmission to a remote station, and to pass successive words to it during reception from a remote station, and on error. Further pluralities of remote stations may be connected to the CPU by their own duplex link, modems and modem controllers. The CPU communicates with only one remote station, per duplex link, at a time. The CPU memory holds inter alia: (a) a receiver area for each duplex link to hold temporarily a received indication message and a "control word" (which is an internal status indicator), (b) a station description table for each remote station specifying site, station address, address in memory of indication words, number of indication words the remote station is expected to transmit, number of control words it expects to receive, status, poll timer, various error counts, &c., (c) a control table for each duplex link to hold information relating to the remote station being communicated with (largely loaded from the station description table), (d) an alarm array, warning array and control array, each with a bit for each remote station, used to indicate expiry of alarm timer (see below), expiry of warning timer (see below) and control request (requirement) pending respectively, (e) for each remote station, an alarm timer (set if an attempt to turn off a failed remote station transmitter which is remaining on the line by sending a zero address fails), a warning timer (set to a lower value than the corresponding poll timer) and an integrity timer (set when the remote station is addressed and used to prevent the duplex link being tied up if the remote station fails to respond in a reasonable time), (f) indication word(s) for each remote station (used for detecting changes in indications from the remote station), (g) control word(s) for each remote station to control apparatus units thereat. The CPU programme initiates communications in response to control requests and expired timers and can cause retry of failed transmissions when it detects error or when a remote station requests retry because it detected error, retry being subject to permitted maximum numbers of tries (entries in control table used). The CPU can decide a remote station is out-of-service and not attempt to communicate with it again until a respective out-of-service timer expires (say 30 minutes), the out-of-service condition being cancelled if the communication attempt is successful. Applied to a railway system, the invention can be used to control track signals and switches, and provide indications of the states of these and of track section occupancy. Application to the electric power industry is also mentioned.